Bidirectional communication module

ABSTRACT

In an example, a communication module includes an optical transmitter, an optical receiver, and a periodical filter. The optical transmitter is configured to emit an outbound optical signal. The optical receiver is configured to receive an inbound optical signal. A first frequency of the outbound optical signal is offset from a second frequency of the inbound optical signal by an amount less than a channel spacing of a multiplexer/demultiplexer implemented in an optical communication system that includes the communication module. The periodical filter is positioned in optical paths of both the outbound optical signal and the inbound optical signal and has a transmission spectrum with periodic transmission peaks and troughs. The first frequency of the outbound optical signal may be aligned to one of the transmission peaks and the second frequency of the inbound optical signal may be aligned to one of the transmission troughs, or vice versa.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalApp. No. 62/311,782, filed Mar. 22, 2016, which is incorporated hereinby reference in its entirety.

FIELD

The embodiments discussed herein are related to bidirectional (bi-di)communication modules and systems.

BACKGROUND

Unless otherwise indicated herein, the materials described herein arenot prior art to the claims in the present application and are notadmitted to be prior art by inclusion in this section.

Some optical communication systems implement wavelength divisionmultiplexing (WDM) in which multiple optical signals on distinctwavelength/frequency channels are transmitted over the same opticalfiber. One WDM architecture is a 2-fiber WDM ring in which a first setof multiple optical signals traveling in one direction, arbitrarilyreferred to herein as eastbound optical signals, are transmitted overone optical fiber, and a second set of multiple optical signalstraveling in an opposite direction, arbitrarily referred to herein aswestbound optical signals, are transmitted over a different opticalfiber. A corresponding multiplexer at an input to each optical fiberspatially combines the eastbound or westbound optical signals fromdifferent communication modules into a corresponding one of the opticalfibers. A corresponding demultiplexer at an output of each optical fiberspatially separates the eastbound or westbound optical signals anddistributes individual optical signals to different communicationmodules.

Some WDM architectures assign the various eastbound/westbound opticalsignals to the ITU-T C-band and/or the ITU-T L-band, each of which canaccommodate 50 channels at 100 gigahertz (GHz) channel spacing. Somelegacy WDM architectures have 100 GHz multiplexers and/ordemultiplexers. Assuming 40 westbound optical signals and 40 eastboundoptical signals in the 2-fiber WDM ring architecture described above,the use of different optical fibers for eastbound versus westboundoptical signals means frequency channels can be re-used across theoptical fibers as long as each frequency channel is only used once peroptical fiber such that all 80 eastbound and westbound optical signalscan be accommodated in the C-band. However, the 2-fiber WDM ringarchitecture requires two separate optical fibers.

Other WDM architectures can be implemented with a single bidirectionaloptical fiber. For instance, if the channel spacing is reduced to 50GHz, all 80 eastbound/westbound optical signals can be accommodated inthe C-band on a single bidirectional optical fiber. Such a configurationrequires a 50 GHz multiplexer/demultiplexer at each end of thebidirectional optical fiber, which may be more costly than 100 GHzmultiplexers/demultiplexers.

Alternatively, the channel spacing for 80 total eastbound/westboundoptical signals in a single bidirectional optical fiber can be kept at100 GHz if channel assignments are extended into the L-band in additionto the C-band. Such an architecture may require a more extensive and/orexpensive inventory of communication modules compared to WDMarchitectures with channel assignments confined to the C-band.

The subject matter claimed herein is not limited to embodiments thatsolve any disadvantages or that operate only in environments such asthose described above. Rather, this background is only provided toillustrate one exemplary technology area where some embodimentsdescribed herein may be practiced.

BRIEF SUMMARY OF SOME EXAMPLE EMBODIMENTS

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential characteristics of the claimed subject matter, nor is itintended to be used as an aid in determining the scope of the claimedsubject matter.

Some example embodiments described herein generally relate to bi-dicommunication modules and systems.

In an example embodiment, a communication module includes an opticaltransmitter, an optical receiver, and a periodical filter. The opticaltransmitter is configured to emit an outbound optical signal. Theoptical receiver is configured to receive an inbound optical signal. Afirst frequency of the outbound optical signal is offset from a secondfrequency of the inbound optical signal by an amount less than a channelspacing of a multiplexer/demultiplexer implemented in an opticalcommunication system that includes the communication module. Theperiodical filter is positioned in optical paths of both the outboundoptical signal and the inbound optical signal and has a transmissionspectrum with periodic transmission peaks and troughs. The firstfrequency of the outbound optical signal may be aligned to one of thetransmission peaks and the second frequency of the inbound opticalsignal may be aligned to one of the transmission troughs, or vice versa.

In another example embodiment, a system includes a localmultiplexer/demultiplexer, multiple local bidirectional communicationmodules, a wavelength monitor, and a centralized controller. The localmultiplexer/demultiplexer includes a fiber-side port and multiplemodule-side ports. The fiber-side port is configured to becommunicatively coupled to one end of an optical fiber having a remotemultiplexer/demultiplexer at an other end of the optical fiber. Thelocal bidirectional communication modules are coupled to the module-sideports of the local multiplexer/demultiplexer. Each of the localbidirectional communication modules is configured to: transmit acorresponding outbound optical signal on a corresponding channel withina corresponding transmission peak of a transmission spectrum of thelocal multiplexer-demultiplexer; and receive a corresponding inboundoptical signal on a corresponding channel within the same correspondingtransmission peak of the transmission spectrum as the correspondingoutbound optical signal. The wavelength monitor is communicativelycoupled to the optical fiber and is configured to monitor wavelengths ofat least one of the inbound optical signals or the outbound opticalsignals. The centralized controller is coupled to the wavelengthmonitor, the local bidirectional communication modules, and multipleremote bidirectional communication modules coupled to module-side portsof the remote multiplexer/demultiplexer. The centralized controller isconfigured, based on wavelength monitor information from the wavelengthmonitor, to control central wavelengths of at least one of the inboundoptical signals or the outbound optical signals

Additional features and advantages of the disclosure will be set forthin the description which follows, and in part will be obvious from thedescription, or may be learned by the practice of the disclosure. Thefeatures and advantages of the disclosure may be realized and obtainedby means of the instruments and combinations particularly pointed out inthe appended claims. These and other features of the present disclosurewill become more fully apparent from the following description andappended claims, or may be learned by the practice of the disclosure asset forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent disclosure, a more particular description of the disclosure willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the disclosure and aretherefore not to be considered limiting of its scope. The disclosurewill be described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 illustrates an example optical communication system thatimplements WDM to communicate multiple optical signals bidirectionallyacross an optical fiber between multiple communication modules;

FIG. 2 illustrates another example optical communication system,arranged in accordance with at least one embodiment described herein;

FIG. 3 illustrates another example optical communication system,arranged in accordance with at least one embodiment described herein;

FIG. 4 summarizes and depicts four different optical communicationsystems A, B, C, and D that implement WDM, arranged in accordance withat least one embodiment described;

FIG. 5 illustrates another example optical communication system,arranged in accordance with at least one embodiment described herein;

FIG. 6 illustrates two example bi-di mux/demux units such as may beimplemented in the optical communication systems of FIGS. 2 and 3,arranged in accordance with at least one embodiment described herein;

FIG. 7 illustrates another example bi-di mux/demux unit such as may beimplemented in the optical communication systems of FIGS. 2 and 3,arranged in accordance with at least one embodiment described herein;

FIG. 8 illustrates another example bi-di mux/demux unit such as may beimplemented in the optical communication systems of FIGS. 2 and 3,arranged in accordance with at least one embodiment described herein;

FIG. 9 illustrates another example bi-di mux/demux unit such as may beimplemented in the optical communication systems of FIGS. 2 and 3,arranged in accordance with at least one embodiment described herein;and

FIG. 10 illustrates another example optical communication systemimplemented with a remote wavelength control scheme.

DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS

FIG. 1 illustrates an example optical communication system 100(hereinafter “system 100”) that implements wavelength divisionmultiplexing (WDM) to communicate multiple optical signalsbidirectionally across an optical fiber 102 between multiplecommunication modules 104, 106. In FIG. 1 and other figures herein, adirection from left to right through the optical fiber 102 isarbitrarily referred to as east, while a direction from right to leftthrough the optical fiber 102 is arbitrarily referred to as west. Eastand west as used herein do not necessarily refer to cardinal directionsbut instead are a convenient shorthand to designate relative directionsand/or orientation of components relative to each other.

In the example of FIG. 1, the system 100 includes 40 communicationmodules 104, 106 at each of two ends of the optical fiber 102. Inparticular, in the example of FIG. 1, the system 100 includes 40communication modules 104 at a west end of the optical fiber 102 and 40communication modules 106 at the east end of the optical fiber 102. Inother examples, the system 100 may include some other number ofcommunication modules 104, 106 at each of the two ends of the opticalfiber 102.

At each end of the optical fiber 102, a first and last of thecommunication modules 104 and 106 (e.g., communication module 1 andcommunication module 40) are depicted and respectively labeled “Bi-DiTRX01” and “Bi-Di TRX40”. Due to space constraints in the drawings,communication modules 2-39 are not illustrated at either end of theoptical fiber 102.

In an example embodiment, each communication module 104, 106 includes atransmitter configured to emit an optical signal that is representativeof an electrical signal received from a host device at a designatedfrequency (and wavelength) that is different than a designated frequency(and wavelength) of other optical signals emitted by transmitters ofother communication modules 104, 106 in the system 100. The variousdesignated frequencies (and corresponding wavelengths) may be referredto as channels. Each communication module additionally includes areceiver configured to receive an optical signal in a particular one ofthe channels.

In FIG. 1 and other Figures herein, each transmitter is labeled “TX” andeach receiver is labeled “RX”. The channel assignments for eachtransmitter and receiver may be labeled according to the format “ChXXY”,where “XX” is a placeholder for a number of the communication module104, 106 in which the transmitter or receiver is included (e.g., “01”for the first communication module 104, 106 or “40” for the lastcommunication module 104, 106) and “Y” is a placeholder for thetransmission direction of the channel (e.g., “A” for eastbound opticalsignals or “B” for westbound optical signals). Thus, the transmitter inthe first communication module 104 at the west end of the optical fiber102 is labeled “TX Ch01A” where TX designates the component as atransmitter and “Ch01A” designates the particular channel “Ch”assignment for the transmitter of the first communication module “01”transmitting in the eastbound direction “A”. The above naming conventionmay analogously be applied to other channel assignments for othertransmitters and receivers in the system 100.

The system 100 additionally includes an opticalmultiplexer/demultiplexer (Mux/Demux) 108, 110 at each end of theoptical fiber 102 between the corresponding end of the optical fiber 102and the corresponding communication modules 104 or 106. In an exampleembodiment, each of the Mux/Demux 108, 110 may include a 100 gigahertz(GHz) Mux/Demux.

In the example of FIG. 1, each Mux/Demux 108, 110 includes 40module-side ports and a single fiber-side port. More generally, eachMux/Demux 108, 110 may include two or more module-side ports, thespecific number of module-side ports depending on the number ofcommunication modules 104, 106 and/or the number of channels in thesystem 100.

In operation, the left Mux/Demux 108 is configured to receive 40eastbound optical signals on its 40 module-side ports from the 40 leftcommunication modules 104 and to spatially combine (e.g., multiplex) the40 eastbound optical signals for output through its fiber-side port tothe optical fiber 102. The 40 spatially combined eastbound opticalsignals are transmitted eastward through the optical fiber 102 to theright Mux/Demux 110. The right Mux/Demux 110 is configured to receivethe 40 spatially combined eastbound optical signals from the opticalfiber 102 through its fiber-side port and to spatially separate (e.g.,demultiplex) out the individual 40 eastbound optical signals. The 40eastbound optical signals are output through the 40 module side ports ofthe right Mux/Demux 110 such that each of the 40 eastbound opticalsignals is provided to a different one of the 40 right communicationmodules 106.

Analogously, the right Mux/Demux 110 is configured to receive 40westbound optical signals on its 40 module-side ports from the 40 rightcommunication modules 106 and to spatially combine (e.g., multiplex) the40 westbound optical signals for output through its fiber-side port tothe optical fiber 102. The 40 spatially combined westbound opticalsignals are transmitted westward through the optical fiber 102 to theleft Mux/Demux 108. The left Mux/Demux 108 is configured to receive the40 spatially combined westbound optical signals from the optical fiber102 through its fiber-side port and to spatially separate (e.g.,demultiplex) out the individual 40 westbound optical signals. The 40westbound optical signals are output through the 40 module side ports ofthe left Mux/Demux 102 such that each of the 40 westbound opticalsignals is provided to a different one of the 40 left communicationmodules 104.

The foregoing example assumes that the left communication modules 104include a total of 40 modules, the right communication modules 106include a total of 40 modules, and each of the left Mux/Demux 108 andthe right Mux/Demux 110 includes 40 fiber-side ports. Embodimentsdescribed herein can analogously be applied to other systems that mayhave a different number of communication modules at opposite ends of anoptical fiber where a Mux/Demux at each end of the optical fiber mayhave a different number of fiber-side ports.

FIG. 1 additionally includes a transmission spectrum 112 of eachMux/Demux 108, 110 along with port and channel assignments in the system100. Each eastbound channel is assigned to a different port of eachMux/Demux 108, 110 and to a different transmission peak of thetransmission spectrum 112 than other eastbound channels. For instance,eastbound channels 01East, 02East, . . . , 39East, and 40East arerespectively assigned to ports Port1, Port2, . . . , Port39, and Port40and to different transmission peaks of the transmission spectrum 112where the transmission peaks have a center-to-center spacing of 100 GHzin one or more embodiments. Similarly, westbound channels 01West,02West, . . . , 39West, and 40West are respectively assigned to portsPort1, Port2, . . . , Port39, and Port40 and to different transmissionpeaks of the transmission spectrum 112 where the transmission peaks havea center-to-center spacing of 100 GHz in one or more embodiments, oraround 100 GHz if we use cyclic AWG.

The relationship between the channel assignment naming convention usedin connection with the naming of the transmitters and receivers asdescribed above and the channel assignments in the transmission spectrum112 is straightforward. For instance, TX Ch01A transmits eastboundchannel 01East, whereas TX Ch10B transmits westbound channel 01West.Analogously, TX Ch40A transmits eastbound channel 04East, whereas TXCh40B transmits westbound channel 04West. The “ChXXY” naming conventionis thus essentially equivalent to the “XXEast” and “XXWest” namingconvention discussed in connection with the transmission spectrum 112.

From FIG. 1 and the foregoing description, it is apparent that twochannels are assigned to each port of each Mux/Demux 108, 110, where oneof the two channels assigned to each port is an eastbound channel andthe other is a westbound channel. It can also be seen from FIG. 1 that aguard band of unused channels may be provided between eastbound channelsand the westbound channels.

Thus, the left Mux/Demux 108 is configured to receive an eastboundoptical signal n (where n is an index from 1-40) emitted by atransmitter n (e.g., TX Ch01A to TX Ch40A) on channel n (e.g., Ch01A toCh40A which correspond to 01East to 40East) of communication module n ofthe communication modules 104 on module-side port n and to spatiallycombine all n eastbound optical signals for output to the optical fiber102 for eastward transmission to the right Mux/Demux 110. The rightMux/Demux 110 receives and spatially separates the n eastbound opticalsignals and outputs each eastbound optical signal n on module-side portn to be received by right communication module n of the communicationmodules 106.

Similarly, the right Mux/Demux 110 is configured to receive a westboundoptical signal n emitted by a transmitter n (e.g., TX Ch10B to TX Ch40B)on channel n (e.g., Ch10B to Ch40B which correspond to 01West to 40West)of communication module n of the communication modules 106 onmodule-side port n and to spatially combine all n westbound opticalsignals for output to the optical fiber 102 for westward transmission tothe left Mux/Demux 108. The left Mux/Demux 108 receives and spatiallyseparates the n westbound optical signals and outputs each westboundoptical signal n on module-side port n to be received by leftcommunication module n of the communication modules 104.

Each communication module 104, 106 in the system 100 may include asingle input/output port through which an outbound optical signalgenerated by the transmitter of the communication module 104, 106 isoutput, and also through which an inbound optical signal received fromthe corresponding Mux/Demux 108, 110 may be received. In these and otherembodiments, each communication module 104, 106 may include a widebandfilter 113 configured to pass the outbound signal and reflect theinbound signal, or vice versa.

In an example embodiment, each wideband filter 113 in the leftcommunication modules 104 may have a transmission spectrum 114. Thetransmission spectrum 114 is designed to be aligned to the transmissionspectrum 112 of each Mux/Demux 108, 110 in FIG. 1. Further, eachwideband filter 113 in the right communication modules 106 may have atransmission spectrum 116. The transmission spectrum 116 is alsodesigned to be aligned to the transmission spectrum 112 of eachMux/Demux 108, 110 in FIG. 1.

As illustrated by the transmission spectra 112 and 114, each widebandfilter 113 in the left communication modules 104 may include a lowpassfilter configured to pass all the eastbound signals on eastboundchannels 1 to 40 (e.g., 01East to 40East) and to reflect all thewestbound channels on westbound channels 1 to 40 (e.g., 01West to40West). For instance, the wideband filter 113 in left communicationmodule 1 of the communication modules 104 may be configured to pass theoptical signal emitted by the transmitter TX Ch01A on eastbound channel01East so that it may be input to the left Mux/Demux 108 through itsmodule-side port 1 and to reflect the optical signal received from theleft Mux/Demux 108 through its module-side port 1 on westbound channel01West to be received by receiver RX Ch01B.

As further illustrated by the transmission spectra 112 and 116, eachwideband filter 113 in the right communication modules 106 may include ahighpass filter designed to pass all the westbound signals on westboundchannels 1 to 40 (e.g., 01West to 40West) and to reflect all theeastbound channels on eastbound channels 1 to 40 (e.g., 01East to40East). For instance, the wideband filter in right communication module1 of the communication modules 106 may be designed to pass the opticalsignal emitted by the transmitter TX Ch01B on westbound channel 01Westso that it may be input to the right Mux/Demux 110 through itsmodule-side port 1 and to reflect the optical signal received from theright Mux/Demux 110 through its module-side port 1 on eastbound channel01East to be received by receiver RX Ch01A.

In an example embodiment of FIG. 1, the eastbound channels may include40 channels with 100 GHz spacing in the ITU-T C-band (e.g., 1530-1565nanometers (nm)) while the westbound channels may include 40 channelswith 100 GHz spacing in the ITU-T L-band (e.g., 1568-1610 nm). The 80total eastbound and westbound channels at 100 GHz spacing with an extrawavelength gap (“Guard band” in FIG. 1) to separate the eastbound andwestbound channels cannot fit in the C-band or the L-band alone.

FIG. 2 illustrates another example optical communication system 200(hereinafter “system 200”), arranged in accordance with at least oneembodiment described herein. Similar to the system 100, the system 200may include a left and right Mux/Demux 208, 210 communicatively coupledby an optical fiber 202, with left communication modules 204 and rightmodules communications 206. Each Mux/Demux 208, 210 in FIG. 2 may be thesame as or similar to the Mux/Demux 108, 110 of FIG. 1 and/or mayinclude a 100 GHz Mux/Demux. Alternatively or additionally, eachMux/Demux 208, 210 may include a cyclic arrayed waveguide grating (AWG),a common AWG, a thin-film filter (TFF), or other suitable Mux/Demux. Asin FIG. 1, in FIG. 2 only some of the communication modules 204, 206 aredepicted due to space constraints. Alternatively or additionally, theleft communication modules 204 may include 40 modules and the rightcommunication modules 206 may include 40 modules.

In the system 100 of FIG. 1, each of the 80 total communication modulesincludes a wideband filter 113 to pass a corresponding outbound opticalsignal emitted by a corresponding transmitter and reflect acorresponding inbound optical signal toward a corresponding receiver. Incomparison, in the system 200 of FIG. 2, each communication module 204,206 may include a bi-di mux/demux 213 with a narrowband cyclic orperiodical filter to accomplish an analogous function. The configurationof FIG. 2 may accommodate a higher channel density that allows all 80channels (assuming 40 left communication modules 204 and 40 rightcommunication modules 206) to be implemented in the C-band withoutrequiring any changes to either of the two Mux/Demux units 208, 210 inthe system 200.

In more detail, FIG. 2 additionally includes the transmission spectrum112 of each Mux/Demux 208, 210 along with port and channel assignmentsin the system 100. Similar to FIG. 1, in FIG. 2, each eastbound channelis assigned to a different port of each Mux/Demux and differenttransmission peak of the transmission spectrum 112 than other eastboundchannels, while each westbound channel is assigned to a different portof each Mux/Demux and different transmission peak of the transmissionspectrum 112 than other westbound channels. However, in FIG. 2, eacheastbound channel is paired with a corresponding westbound channel wherethe two channels in each pair may be spaced by tens of GHz, and both theeastbound channel and the westbound channel are not only assigned to thesame port (as in FIG. 1), but are also assigned to the same transmissionpeak of the transmission spectrum 112. In FIG. 2, the eastbound channelsmay have about 100 GHz spacing between adjacent eastbound channels,while the westbound channels may also have about 100 GHz spacing betweenadjacent westbound channels. The spacing between the eastbound andwestbound channel in each pair can be 50 GHz, or generally somewherebetween 30 GHz and 70 GHz. More generally, assuming the transmissionspectrum 112 of each Mux/Demux 208, 210 has transmission peaks with acenter-to-center spacing between adjacent transmission peaks of Δ GHz,the spacing between the eastbound and westbound channel in each pair canbe 0.5*Δ GHz or in a range from 0.3*Δ GHz to 0.7*Δ GHz.

By pairing the eastbound and westbound channels together at tens of GHzspacing between the two channels of the pair, all 80 channels of FIG. 2can be accommodated in the C-band without replacing legacy componentssuch as each of the 100 GHz Mux/Demux units 208, 210 in this example. InFIG. 2, each eastbound channel in a pair is illustrated as being at afrequency tens of GHz lower than the westbound channel in the pair. Inother embodiments, the arrangement is reversed with each westboundchannel in the pair being at a frequency tens of GHz lower than theeastbound channel in the pair.

FIG. 3 illustrates another example optical communication system 300(hereinafter “system 300”), arranged in accordance with at least oneembodiment described herein. The system 300 is similar in some respectsto the system 200 of FIG. 2, except that in the system 300, each of aleft and right Mux/Demux 308, 310 may include a 400 GHz or coarsewavelength division multiplexing (CWDM) Mux/Demux instead of the example100 GHz Mux/Demux 208, 210 discussed in the example of FIG. 2.

Each of the communication modules 204, 206 in the system 300 may includethe bi-di mux/demux 213 with a narrowband cyclic or periodical filter,as discussed with respect to FIG. 2. Additional details regardingexample bi-di mux/demux units that may be implemented in thecommunication modules 204, 206 of FIGS. 2 and 3 are described in moredetail with respect to, e.g., FIGS. 6-9.

Each of the left and right Mux/Demux 308, 310 of FIG. 3 includes atransmission spectrum 312 with, e.g., about 400 GHz channels or about800 GHz channels. In the example of FIG. 3, multiple eastbound channelsand multiple westbound channels may be assigned to the same module-sideport of the Mux/Demux 308, 310 in an alternating arrangement where eachpair of adjacent channels is separated by tens of GHz. For instance, inthe example of FIG. 3, each Mux/Demux 308, 310 includes ten ports (e.g.,“Port 1”, . . . , “Port10”) with four eastbound channels and fourwestbound channels assigned to each port (e.g., “1aEast”, “1aWest”,“1bEast”, “1bWest”, “1cEast”, “1cWest”, “1dEast”, and “1dWest” are allassigned to port 1). The eastbound and westbound channels may bearranged in pairs (as described with respect to FIG. 2), where each pairis referred to as a subport, including for port 1, subports “Port1a”,“Port1b”, “Port1c”, and “Port1d”. Each subport in the example of FIG. 3may be associated with a different communication module 204, 206 suchthat each port may be associated with as many communication modules asthe port has subports.

The system 300 may additionally include a beam splitter 315 for eachmodule-side port of each Mux/Demux 308, 310 between the correspondingport and the communication modules 204, 206 associated with that port.In FIG. 3, a single beam splitter 315 is illustrated at each end of theoptical fiber 202 for simplicity with the understanding that at each endof the optical fiber 202, the system 300 may include as many beamsplitters 315 as ports (e.g., a different beam splitter 315 coupled toeach port) of the corresponding Mux/Demux 308, 310. Each beam splitter315 may limit exchange of optical signals between each communicationmodule 204, 206 and the corresponding Mux/Demux 308, 310 to thecorresponding pair of eastbound and westbound channels, or thecorresponding subport, associated with each communication module 204,206. As such, each beam splitter 315 may include a 100 GHz Mux/Demux orother suitable optical device in this example.

FIG. 4 summarizes and depicts four different optical communicationsystems (hereinafter “system” or “systems’) A, B, C, and D thatimplement WDM, arranged in accordance with at least one embodimentdescribed herein. FIG. 4 additionally includes a table summarizingvarious parameters associated with each of the systems A-D. The system100 of FIG. 1 is an example implementation of the system C in FIG. 4.The system 200, 300 of FIG. 2 or 3 is each individually an exampleimplementation of the System D in FIG. 4. Other systems described insubsequent Figures are also examples of the System D in FIG. 4.

As suggested by FIG. 4, embodiments of system D may have a lower costthan the systems A-C since the system D can be implemented with one feedfiber, and fewer terminations. Embodiments of the system D may also useexisting low cost 100 GHz mux/demux units without the need for C+L bandcyclic AWG and L-band modules. Alternatively or additionally,embodiments of the system D of FIG. 4 may enable flexible architecturesthat allow any combinations of optical add drop multiplexers (OADMs),multiplexers (Mux), and demultiplexers (Demux) without being limited byavailable 50 GHz mux/demux or C+L cyclic AWG. In addition, the system Dcan fit more channels, e.g., into standard C-band, as compared to thesystem C, as there's no need to reserve wavelength gap. An example ofsuch an architecture that may be enabled by embodiments of the system Dis illustrated in FIG. 5. In FIG. 5, “RRU” stands for remote radio unitand “BBU” stands for base band unit.

Embodiments of the bi-di mux/demux that may be implemented in each ofthe communication modules 204, 206 of the system 200, 300, or of thesystem D of FIGS. 2-4 may have any of a variety of configurations and/ormay include a fixed or tunable cyclic or periodical filter. Variousexample configurations of a bi-di mux/demux are illustrated anddescribed in more detail below.

For instance, FIG. 6 illustrates two example bi-di mux/demux units 600A,600B such as may be implemented in the system D, the system 200, thesystem 300, and/or other systems described herein, arranged inaccordance with at least one embodiment described herein. In each of thebi-di mux/demux units 600A, 600B, components of the bi-di mux/demux unit600A or 600B are surrounded by a dashed outline, and other components ofthe corresponding communication module or corresponding system aredepicted outside the outline for context. The components external to thebi-di mux/demux units 600A, 600B include a transmitter (TX laser), areceiver (RX PD), and a fiber. Other bi-di mux/demux units describedherein are similalry depicted in relation to a transmitter, a receiver,and a fiber for context, and the description of such components will notbe repeated except as needed to describe operation of the correspondingbi-di mux/demux units.

Each of the bi-di mux units 600A, 600B includes a first filter 602arranged at any suitable angle, such as 45 degrees to the fiber, and anoptical isolator 604 positioned between the transmitter and the firstfilter 602. Each bi-di mux unit 600A, 600B additionally includes a firstlens 606 positioned between the transmitter and the optical isolator604, a second lens 608 positioned between the receiver and the firstfilter 602, and a third lens 610 positioned between the first filter 602and the fiber. The first lens 606 between the transmitter and theoptical isolator 604 may be configured to collimate an outbound opticalsignal emitted by the transmitter. The second lens 608 between thereceiver and the first filter 602 may be configured to focus an inboundoptical signal onto the receiver. The third lens 610 may be configuredto focus the outbound optical signal received from the transmitter(through the first lens 606, the optical isolator 604, and the firstfilter 602) into the fiber. The third lens 610 may also be configured tocollimate the inbound optical signal received from the fiber, whichcollimated inbound optical signal is directed to the first filter 602,which redirects the collimated inbound optical signal to the receiverthrough the second lens 608 and through a second filter 612 in the bi-dimux/demux unit 600B. Other bi-di mux/demux units described herein maysimilarly include first, second, and third lenses and an opticalisolator and the description of such components will not be repeatedexcept as needed to describe operation of the corresponding bi-dimux/demux units. The bi-di mux/demux unit 600B additionally includes thesecond filter 612 between the first filter 602 and the receiver, andmore particularly between the first filter 602 and the second lens 608.

The first filter 602 in each of the bi-di mux/demux units 600A, 600B mayinclude a non-flat top transmission spectrum 602A. Alternatively, thefirst filter 602 in each of the bi-di mux/demux units 600A, 600B mayinclude a flat-top transmission spectrum 602B. In the graphs thatinclude the transmission spectra 602A and 602B, vertical lines delimitboundaries of 100 GHz channels in, e.g., the ITU-T C-band grid withrespect to the transmission spectra 602A, 602B.

As illustrated in FIG. 6, each of the transmission spectra 602A and 602Bmay include a first free spectral range (FSR1) of 100 GHz, or some otherFSR1. In some embodiments, the outbound optical signal emitted by eachof the transmitters in FIG. 6 may be at a channel positioned or alignedto a transmission peak of the transmission spectrum 602A or 602B so thatthe outbound optical signal may pass through the first filter 602. Incomparison, the inbound optical signal may be at a channel positioned oraligned to a trough of the transmission spectrum 602A or 602B so thatthe inbound optical signal may be reflected by the first filter 602toward the receiver.

The second filter 612 in the bi-di mux/demux unit 600B may include atransmission spectrum 604A when the first filter 602 includes thenon-flat top transmission spectrum 602A. Alternatively or additionally,the second filter 612 in the bi-di mux/demux unit 600B may include atransmission spectrum 604B when the first filter 602 includes theflat-top transmission spectrum 602B. Each of the transmission spectra604A and 604B may include a second free spectral range (FSR2) of 100GHz, or some other FSR2. In some embodiments, the inbound optical signalreceived by the bi-di mux/demux unit 600B may be at a channel aligned toa transmission peak of the transmission spectrum 604A or 604B so thatthe inbound optical signal may pass through the second filter 612 toreach the receiver.

The first filter 602 in the bi-di mux/demux unit 600A may include afixed (e.g., non-tunable) filter. The first and second filter 602, 612in the bi-di mux/demux unit 600B may also each include a fixed filter.In other embodiments, one or both of the first and second filter 602,612 may include a tunable filter, as described in more detail below.

FIGS. 7-9 illustrate three other example bi-di mux/demux units 700, 800,900 such as may be implemented in the system D, the system 200, thesystem 300, and/or other systems described herein, arranged inaccordance with at least one embodiment described herein. The bi-dimux/demux unit 700 of FIG. 7 includes the same or similar components asthe bi-di mux/demux unit 600B of FIG. 6, except that the bi-di mux/demuxunit 700 of FIG. 7 includes first and second tunable filters 702, 712instead of fixed first and second filters 602, 612. The bi-di mux/demuxunit 700 of FIG. 7 may be implemented in a communication module with acontroller and/or one or more sensors, hereinafter Detect and Control,to tune each of the first and second tunable filters 702, 712 asdesired.

The first tunable filter 702 in the bi-di mux/demux unit 700 may havethe transmission spectrum 602A or 602B, which transmission spectrum 602Aor 602B may be tunable. Similarly, the second tunable filter 712 in thebi-di mux/demux unit 700 may have the transmission spectrum 604A or604B, which transmission spectrum 604A or 604B may be tunable.

The bi-di mux/demux unit 800 of FIG. 8 includes the same or similarcomponents as the bi-di mux/demux unit 700 of FIG. 7 and additionallyincludes a tap splitter 814 positioned between the third lens 610 andthe first tunable filter 702 and a monitor photodiode (MPD) 816. The tapsplitter 814 may be configured to reflect about 5% or some otherrelatively small amount of the outbound optical signal toward the MPD816 for monitoring purposes. Similar to the bi-di mux/demux unit 700 ofFIG. 7, the bi-di mux/demux unit 800 of FIG. 8 may be implemented in acommunication module with Detect and Control.

The bi-di mux/demux unit 900 of FIG. 9 includes the same or similarcomponents as the bi-di mux/demux unit 800 of FIG. 8, although in aslightly different arrangement. In particular, the tap splitter 814 ispositioned between the first tunable filter 702 and the optical isolator604 in the bi-di mux/demux unit 900 of FIG. 9. Similar to the bi-dimux/demux units 700, 800 of FIGS. 7 and 8, the bi-di mux/demux unit 900of FIG. 9 may be implemented in a communication module with Detect andControl.

The first filter 602, 702 in each of the bi-di mux/demux units 600A,600B, 700, 800, 900 described herein may include a periodical filter toprovide add/drop of one channel in a DWDM channel spacing. In someembodiments, the transmitter included in a corresponding communicationmodule with each bi-di mux/demux unit 600A, 600B, 700, 800, 900 may betunable so that each communication module may be used for any of theinbound/outbound channel pairs. Thus, any replacement communicationmodules kept on hand in the event of failure may replace any failedcommunication module.

As already mentioned, FSR1 and/or FSR2 of the transmission spectra 602A,602B, 604A, 604B of the first filters 602, 702 and the second filters612, 712 in the bi-di mux/demux units 600A, 600B, 700, 800, 900 may be100 GHz, or more generally may be the same as ITU-T channel spacing whenthe first filters 602, 702 and the second filters 612, 712 includeetalon-based fixed wavelength filters. Alternatively, FSR1 and/or andFSR2 may be 1×, 2×, or multiple channel spacing when the first filters602, 702 and the second filters 612, 712 include tunable filters,provided the tuning range of the first filters 602, 702 and the secondfilters 612, 712 can cover all DWDM channels considering its periodicalnature.

For non-flat-top tunable filter based architecture, one embodiment is tolock the second tunable filter 712 to a remote transmitter so that thepower to a local receiver can reach or at least tend toward maximum,while the first tunable filter 702 can lock to the central wavelength ofa local transmitter by monitoring the MPD 816 and keep it as maximum, orrelatively close to maximum.

Alternatively or additionally, the second tunable filter 712 can be usedas an etalon for control of a central wavelength of a remote transmitterby monitoring the power to the local receiver and keep it as maximum, orrelatively close to maximum. The peak wavelength of the second tunablefilter 712 as an etalon can be set by tuning temperature of the secondtunable filter 712 to achieve a target wavelength. In these and otherembodiments, the Bi-Di communication modules such as 204 and 206 inFIGS. 2 and 3 at local and remote sites may be capable of havingconnection with each other. One way to implement the remote control isto use in-band communication channel to exchange wavelength informationas defined in ITU-T G.metro. In another embodiment, the first tunablefilter 702 can be used as an etalon for control of a central wavelengthof the local transmitter by monitoring the MPD 816 and keep it asmaximum, or relatively close to maximum. The peak wavelength of thefirst tunable filter 702 as an etalon can be set by tuning temperatureof the first tunable filter 702 to achieve a target wavelength.

For a flat-top tunable filter based architecture, one embodiment is tolock both the first tunable filter 702 and the second tunable filter 712to a remote transmitter, e.g., by locking the first tunable filter 702first and by locking the second tunable filter 712 second, so that thepower to the local receiver can reach maximum. In the local transmitdirection, the first tunable filter 702 may be tuned according to theremote transmitter while the local transmitter output power can keepsame due to the flat-top architecture.

One or more embodiments of the tunable Bi-Di architecture describedherein can also work together with a remote wavelength control scheme1000, as illustrated in FIG. 10. A wavelength monitor 1002 monitorswavelength of all remote channels, or monitors all local channels aswell. Bi-Di communication modules at remote sites may have a remoteconnection to a centralized controller 1004. One way to implement theremote control is to use in-band communication channel to exchangewavelength information as defined in ITU-T G.metro. The centralizedcontroller 1004 can communicate with Bi-Di communication modules at bothlocal site and remote site and control wavelength of each Bi-Dicommunication module according to the wavelength monitor informationuntil achieving a corresponding target wavelength for each Bi-Dicommunication module. For this scenario, the first tunable filter 702and/or the second tunable filter 712 may track to the central wavelengthof a corresponding transmitter.

Some embodiments described herein include a first filter 602, 702 thatis transmissive to an outbound optical signal emitted by a transmitterand that reflects an inbound optical signal toward a receiver. In otherembodiments, the first filter 602, 702 may be transmissive to theinbound optical signal and may reflect the outbound optical signal, inwhich case the positions of the transmitter and the receiver may beswitched as compared to the embodiments illustrated in the figures.

Additional details and examples are included in the Appendix filedherewith, which is incorporated herein by reference.

All examples and conditional language recited herein are intended forpedagogical objects to aid the reader in understanding the presentdisclosure and the concepts contributed by the inventor to furtheringthe art, and are to be construed as being without limitation to suchspecifically recited examples and conditions. Although embodiments ofthe present disclosure have been described in detail, it should beunderstood that the various changes, substitutions, and alterationscould be made hereto without departing from the spirit and scope of thepresent disclosure.

The invention claimed is:
 1. A communication module comprising: anoptical transmitter configured to emit an outbound optical signalrepresentative of an outbound electrical signal received from a host,the outbound optical signal transmitted into an optical communicationsystem through a module-side port of a multiplexer/demultiplexer of theoptical communication system; an optical receiver configured to receivean inbound optical signal through the same module-side port of themultiplexer/demultiplexer through which the outbound optical signal istransmitted and to convert the inbound optical signal into an inboundelectrical signal to provide to the host, wherein a first frequency ofthe outbound optical signal transmitted through the module-side port ofthe multiplexer/demultiplexer is offset from a second frequency of theinbound optical signal received through the module-side port of themultiplexer/demultiplexer by an amount less than a channel spacing ofthe multiplexer/demultiplexer, the first frequency of the outboundoptical signal and the second frequency of the inbound optical signaleach aligned to a common transmission peak of a transmission spectrum ofthe multiplexer/demultiplexer; and a periodical filter positioned in anoptical path of the outbound optical signal and in an optical path ofthe inbound optical signal, the periodical filter having a transmissionspectrum with periodic transmission peaks and troughs, wherein: thefirst frequency of the outbound optical signal is aligned to one of thetransmission peaks and the second frequency of the inbound opticalsignal is aligned to one of the transmission troughs; or the firstfrequency of the outbound optical signal is aligned to one of thetransmission troughs and the second frequency of the inbound opticalsignal is aligned to one of the transmission peaks.
 2. The communicationmodule of claim 1, wherein the periodical filter includes a fixedwavelength filter with fixed periodic transmission peaks and troughs. 3.The communication module of claim 2, wherein the fixed wavelength filterincludes an etalon-based fixed wavelength filter.
 4. The communicationmodule of claim 1, wherein the periodical filter includes a tunablewavelength filter with tunable periodic transmission peaks and troughs.5. The communication module of claim 4, wherein the tunable wavelengthfilter includes an etalon-based tunable wavelength filter.
 6. Thecommunication module of claim 5, wherein a peak wavelength of theetalon-based tunable wavelength filter is configured to be set by tuningtemperature of the etalon-based tunable wavelength filter to achieve atarget wavelength.
 7. The communication module of claim 1, wherein theperiodical filter includes a tunable filter to provide add/drop of theoutbound optical signal and the inbound optical signal within thechannel spacing of the multiplexer/demultiplexer.
 8. The communicationmodule of claim 1, wherein the optical transmitter includes a tunabletransmitter.
 9. The communication module of claim 1, further comprising:a tap splitter positioned in the optical path of the outbound opticalsignal to tap a portion of the outbound optical signal to redirect theportion of the outbound optical signal to a monitor optical path; amonitor photodiode positioned in the monitor optical path to receive theportion of the outbound optical signal and measure a power of theoutbound optical signal; and a processor coupled to the opticaltransmitter, the optical receiver, and the periodical filter andconfigured to tune the periodical filter, monitor the measured power ofthe outbound optical signal, and monitor power of the inbound opticalsignal measured by the optical receiver.
 10. The communication module ofclaim 9, wherein the processor is further configured to adjust, relativeto each other, at least one of a central emission wavelength of theoptical transmitter or a central wavelength of a transmission peak ofthe transmission spectrum of the periodical filter to increase themeasured power of the outbound optical signal.
 11. The communicationmodule of claim 1, wherein: the periodical filter comprises a firstperiodical filter; the communication module further comprising a secondperiodical filter positioned in an optical path of the inbound opticalsignal between the optical receiver and the first periodical filter; andthe second periodical filter having a transmission spectrum withperiodic transmission peaks and troughs respectively aligned to theperiodic transmission troughs and peaks of the transmission spectrum ofthe first periodical filter.
 12. The communication module of claim 11,wherein one or both of the first periodical filter or the secondperiodical filter is configured to be used to control a centralwavelength of the optical transmitter as a local optical transmitter orof a remote optical transmitter that generates the inbound opticalsignal.
 13. The communication module of claim 1, wherein the periodicalfilter comprises a non-flat top filter.
 14. The communication module ofclaim 1, wherein the periodical filter comprises a flat top filter. 15.A system comprising: a local multiplexer/demultiplexer that includes afiber-side port and a plurality of module-side ports, wherein thefiber-side port is configured to be communicatively coupled to one endof an optical fiber having a remote multiplexer/demultiplexer at another end of the optical fiber; a plurality of local bidirectionalcommunication modules coupled to the plurality of module-side ports ofthe local multiplexer/demultiplexer, each of the plurality of localbidirectional communication modules configured to: transmit through acorresponding module-side port of the local multiplexer/demultiplexer acorresponding outbound optical signal on a corresponding channel withina corresponding transmission peak of a transmission spectrum of thelocal multiplexer-demultiplexer; and receive a corresponding inboundoptical signal through the same corresponding module-side port of thelocal multiplexer-demultiplexer as the corresponding outbound opticalsignal, the corresponding inbound optical signal received on a differentcorresponding channel within the same corresponding transmission peak ofthe transmission spectrum as the corresponding outbound optical signal;a wavelength monitor communicatively coupled to the optical fiber andconfigured to monitor wavelengths of at least one of the inbound opticalsignals or the outbound optical signals; and a centralized controllercoupled to the wavelength monitor, the plurality of local bidirectionalcommunication modules, and a plurality of remote bidirectionalcommunication modules coupled to module-side ports of the remotemultiplexer/demultiplexer; wherein the centralized controller, based onwavelength monitor information from the wavelength monitor, isconfigured to control central wavelengths of at least one of the inboundoptical signals or the outbound optical signals; and wherein each of theplurality of local bidirectional communication modules includes aperiodical filter positioned in an optical path of the correspondingoutbound optical signal and in an optical path of the correspondinginbound optical signal, the periodical filter having a transmissionspectrum with periodic transmission peaks and troughs.
 16. The system ofclaim 15, wherein each of the local communication modules includes: anoptical transmitter configured to emit a corresponding one of theoutbound optical signals; and an optical receiver configured to receivea corresponding one of the inbound optical signals and to convert thecorresponding one of the inbound optical signals into a correspondinginbound electrical signal; wherein a first frequency of thecorresponding one of the outbound optical signals is offset from asecond frequency of the corresponding one of the inbound optical signalsby an amount less than a channel spacing of the localmultiplexer/demultiplexer; and wherein: the first frequency of thecorresponding one of the outbound optical signals is aligned to one ofthe transmission peaks and the second frequency of the corresponding oneof the inbound optical signals is aligned to one of the transmissiontroughs; or the first frequency of the corresponding one of the outboundoptical signals is aligned to one of the transmission troughs and thesecond frequency of the corresponding one of the inbound optical signalsis aligned to one of the transmission peaks.
 17. The system of claim 16,wherein the periodical filter of each of the local communication modulesis configured to track to a central wavelength of the opticaltransmitter of the corresponding local communication module.
 18. Thesystem of claim 17, wherein: the periodical filter of each of the localcommunication modules comprises a first periodical filter of thecorresponding local communication module; each of the localcommunication modules further comprises a second periodical filterpositioned in an optical path of the corresponding inbound opticalsignal between the corresponding optical receiver and the correspondingfirst periodical filter; and the second periodical filter having atransmission spectrum with periodic transmission peaks and troughsrespectively aligned to the periodic transmission troughs and peaks ofthe transmission spectrum of the corresponding first periodical filter.19. The system of claim 18, wherein the second periodical filter of eachof the local communication modules is configured to track to a centralwavelength of an optical transmitter of a corresponding remotecommunication module that generates the corresponding inbound opticalsignal.
 20. The system of claim 15, wherein: the localmultiplexer/demultiplexer comprises a 100 gigahertz (GHz)multiplexer/demultiplexer; the plurality of local bidirectionalcommunication modules includes forty local bidirectional communicationmodules configured to collectively generate forty outbound opticalsignals, each on a different channel than each other; the plurality ofremote bidirectional communication modules includes forty remotebidirectional communication modules configured to collectively generateforty inbound optical signals, each on a different channel than eachother and than the forty outbound optical signals; and the channels ofthe forty outbound optical signals and the forty inbound optical signalsall fall within a wavelength range of the ITU-T C-band from 1530-1565nanometers.
 21. The system of claim 20, wherein: the localmultiplexer/demultiplexer has a spectral port center-to-port centerspacing of A GHz; and for each of the local communication modules, thedifferent channels of the corresponding outbound optical signal outputfrom the local communication module and of the corresponding inboundoptical signal received at the local communication module are spectrallyseparated from each other by an amount in a range from 0.3*A GHz and0.7*A GHz.
 22. A system comprising: a local multiplexer/demultiplexerthat includes a fiber-side port and a plurality of module-side ports,wherein the fiber-side port is configured to be communicatively coupledto one end of an optical fiber having a remote multiplexer/demultiplexerat an other end of the optical fiber; a plurality of local bidirectionalcommunication modules coupled to the plurality of module-side ports ofthe local multiplexer/demultiplexer, each of the plurality of localbidirectional communication modules configured to: transmit through acorresponding module-side port of the local multiplexer/demultiplexer acorresponding outbound optical signal on a corresponding outboundchannel within a corresponding transmission peak of a transmissionspectrum of the local multiplexer-demultiplexer; and receive acorresponding inbound optical signal through the same correspondingmodule-side port of the local multiplexer-demultiplexer as thecorresponding outbound optical signal, the corresponding inbound opticalsignal received on a corresponding inbound channel within the samecorresponding transmission peak of the transmission spectrum as thecorresponding outbound optical signal; wherein the corresponding inboundchannel is different than the corresponding outbound channel; whereineach of the local communication modules includes a periodical filterpositioned in an optical path of the corresponding outbound opticalsignal and in an optical path of the corresponding inbound opticalsignal, the periodical filter having a transmission spectrum withperiodic transmission peaks and troughs; and wherein: a first frequencyof the corresponding outbound optical signal is aligned to one of thetransmission peaks and a second frequency of the corresponding inboundoptical signal is aligned to one of the transmission troughs; or thefirst frequency of the corresponding outbound optical signal is alignedto one of the transmission troughs and the second frequency of thecorresponding inbound optical signal is aligned to one of thetransmission peaks.